Key properties of films include tear strength and impact resistance, and it is well known that these two parameters generally have an inverse correlation, such that an improvement in one of them is accompanied by a deterioration in the other. Another key property is the processability of the composition from which the films are made, specifically in terms of extrudability and bubble stability.
It is known to make films from bimodal polyethylene compositions, that is to say compositions comprising a low molecular weight (LMW) component and a high molecular weight (HMW) component. The presence of a lower molecular weight fraction can improve the extrudability of the resin, whereas the presence of a higher molecular weight fraction ensures good mechanical properties and melt strength. However films made from bimodal resins may have an inferior appearance due to the presence of gels, which is indicative for a lower degree of homogeneity of the final resin.
Our own WO 2006/018245 discloses polyethylene film compositions having a particular relationship between melt storage modulus G′, measured in Pa and at a dynamic frequency where the loss modulus G″=3000 Pa, and dynamic complex viscosity η*100, measured in Pa·s at 100 rad/s. Storage modulus G′ (at a loss modulus G″ of 3000 Pa), also referred as G′(G″=3000), which is linked to the long chain branching content and the breadth of the molecular weight distribution of the resin, affects the melt elasticity and melt strength of a molten resin during blown film extrusion; a higher G′ corresponds to a higher melt strength. This is desirable for film blowing, as a high melt strength gives improved bubble stability. However if G′ is too high mechanical properties can be adversely affected. No minimum G′ is specified in WO 2006/018245, but the lowest exemplified is 1810 Pa. Impact and tear strength are given for some of the Examples in WO 2006/018245, but there is no disclosure relating to a specific relationship between the two.
EP 1712574A discloses a polyethylene resin for films said to have improved processability and mechanical properties, particularly impact strength. Although the resin is simply stated to have a density of 940 kg/m3 or more, the Examples all have densities in the range 945-947 kg/m3, as it is well known that dart drop impact resistance declines sharply with increasing density (see for example Ster van der Ven, “Polypropylene and other Polyolefins”, Elsevier Science Publishers, 1990, p. 489). However, the low density exemplified in EP 1712574A would also be expected to result in lower barrier and tensile properties as a result of the lower crystallinity at decreased density (A J Peacock, Polyethylene Handbook 2000, page 132, 190).